In commonly owned, copending U.S. application Ser. No. 577,466, filed on Feb. 6, 1984, as "PVF.sub.2 Film Having Dielectric Constants and Capacitors and Camera Flash Apparatus Incorporating Same" now abandoned in favor of continuation application U.S. Ser. No. 833,431 filed on Feb. 24, 1986, there is described an improved polyvinylidene fluoride film that is particularly useful in making capacitors. For such use, the film requires a high dielectric constant and thicknesses no greater than about 5 .mu.m. Such thinness is used to achieve volumetric efficiencies, that is, to allow wrapping of the electrode-covered film to achieve a maximum number of capacitive plates per transverse thickness.
The film described in the aforesaid application has been very effective in providing the above-noted criteria. However, such film is predominantly alpha phase only, and while having sufficient mechanical strength, it does not have a mechanical strength that is superior. As a result, more care is required in handling such film than would be the case if superior mechanical strengths, such as occur in beta phase film, could be achieved. Furthermore, the production of only alpha phase has meant that the PVF.sub.2 film could not be used, as manufactured, for piezoelectric or pyroelectric properties, which require predominantly beta phase crystalline structure.
It has long been known that alpha phase PVF.sub.2 film is convertable into beta phase film by orienting the film such as by stretching it, either uniaxially or biaxially. Alternatively, U.S. Pat. Nos. 4,392,178 and 4,365,283 teach that conversion from alpha phase to beta phase can can achieved by exposing the already cast PVF.sub.2 film to corona discharge. However, such techniques are time-consuming, particularly if they require biaxial stretching.
Therefore, prior to this invention there has been a need for cast or extruded PVF.sub.2 film which is predominantly beta phase film as cast or extruded, without further processing. As used herein, "predominantly" means, at least 75% by weight of the crystalline structure. Prior to this invention, there was no known method for achieving such a result.